The present invention relates to an optical communication system, and more particularly to an optical communication system feasible for the satellite communication architecture today.
Nowadays, microwave is the popular carrier wave for signal transmission used in the current satellite communication system. Taking an example of the Ka band, the message bandwidth of the Ka band applied in practical system is limited up to 155 Mbit/s. In addition, there will be a great deal of power consumed in the microwave transmission. Therefore, the signal has to be transmitted in packets as transmitting a large quantity of digital data. Accordingly, conventional microwave satellite signal transmission will cost considerable time and a large expenditure. With the rapid increase of the transmitted data quantity, the microwave communication gradually can not meet with the high-capacity data transmission requirements in the future. In particular, the bandwidth of optical fiber communication backbone on the ground has reached 40 Gbit/s, which is far larger than the bandwidth of microwave communication. Thus, if the coming satellite communication is intended to be combined with the ground optical fiber communication system for the purpose of constructing the global high speed communication backbone network, it is in an urgent need to enlarge the data transmission quantity. It would be apparent that using laser light as the carrier signal in a satellite communication system will become an inevitable trend for carrying large quantity data.
With relation to the optical communication techniques of the prior art, S. Arnon and N. S. Kopeika presented a design about an adaptive optical transmitter and receiver for space communication through thin clouds in order to overcome the interference caused by the cloud, in xe2x80x9cApplied Opticsxe2x80x9d, Volume 36, No. 9, 1997, p. 1987-1993. Such a design requires that the information about cloud""s variation must be acquired in advance so as to adjust the optical transmitter and receiver. Unfortunately, the acquisition of the information about cloud would be a very tough task. Moreover, the design is focused on the effect of weather, but other factors such as transmitter, satellite, receiver and so on are not taken into consideration.
S. Arnon, S. Rotman and N. S. Kopeika proposed a method for minimizing the bit error rate resulting from the satellite vibration, in xe2x80x9cIEEE Transaction on Aerospace and Electronic Systemsxe2x80x9d, Volume 34, No. 2, 1998, p. 590-596. The method calculates the bit error rate of the demodulated signal in a calculating unit for adjusting the system parameter and minimizing the bit error rate. However, this proposal did not explore the weather issues and frequency drifting effect, and hence it is not applicable to the practical communications emerging between satellite and the earth.
In 1995, Van Deventer proposes a new scheme of optical mixing device having one photodetector for a heterodyne receiver in order to eliminate the optical polarization phenomenon, in U.S. Pat. No. 5,457,563. This scheme attaches two additional optical mixers to the transmitting end and separates the signal into two packets for transmitting these two packets through two different transmission paths. Subsequently, a photodetector and a heterodyne receiver are attached to the receiving end in order to demodulate the signal. Because these two packets are emitted from the same signal source, the optical polarization phenomenon can be eliminated by means of the signal time delay resulting from applying two different transmission paths. Consequently, this scheme is effective on condition the laser is in stable state.
Horiuchi et al. disclose a coherent optical receiver having optically amplified local oscillator signal, in U.S. Pat. No. 5,463,461. The coherent optical receiver is characterized in that an optical amplifier is employed to amplify a local oscillating laser""s power and a heterodyne receiver consisting of a combination of an optical coupler and a photodetector is used to amplify the received weak signal. Hereinafter, an electrical signal amplifier stage is attached to the output end of the photodetector. The advantage of this invention is that the advantage of using a high-power laser can be obtained without boosting laser""s emitting power. Nevertheless, this invention does not deal with the drawback of laser""s unstable frequency, and thus it will be uneasy to demodulate the modulated signal accurately.
Taneya et al. disclose a system for emitting a beam in the air, in U.S. Pat. No. 5,457,561. The system utilizes a transmitting device to mix the local oscillating laser signal with the emitted signal and then the mixed signal is propagated to a frequency discriminating circuit to generate a differential frequency. Thereafter, a control circuit and a wavelength variable laser element are employed to modulate the emitted signal. Because the local oscillating laser at the receiving end is the same as the laser element for modulating the emitted signal, the modulated signal can be accurately demodulated. Because the system utilizes laser""s wavelength to modulate the signal, it will not be usable on the condition of the laser""s frequency drifting. Besides, the system is designed for the purpose of short-distance communication, it is also not suitable for satellite communication.
However, the oscillator laser light frequency drifting is very sensitive to the temperature variation. Besides, while the signal is propagating from the transmitting end to the receiving end, it will encounter lots of impacts from the atmosphere, and thus the optical receiver of a conventional coherent optical communication system is not easy to lock the received signal during the demodulation process. That would seriously affect the demodulation ability.
It is therefore tried by the applicant to deal with the above situations encountered by the prior art.
It is an object of the present invention to develop an optical communication system for minimizing the bit error rate.
It is another object of the present invention to develop an optical communication system for improving the data transmission quantity.
It is another further object of the present invention to develop an optical communication system for resolving the problem of laser frequency drifting.
It is still an object of the present invention to develop a method for communication with an optical signal.
In accordance with a first aspect of the present invention, the optical communication system includes an optical transmitting device for emitting an optical signal carrying an input data as a carrier signal, a first reference optical signal generator for emitting a first reference optical signal, wherein the first reference optical signal has a first beat frequency difference with the frequency of the carrier signal, an optical receiving device receiving the carrier signal and the first reference optical signal through an optical transmission path for retrieving the input data, and a second reference optical signal generator for emitting a second reference optical signal to the optical receiving device, wherein the frequency of the second reference optical signal can be adjusted according to the first beat frequency difference and a second frequency difference which is a frequency difference between the second reference optical signal frequency generated in the second reference optical signal generator and the carrier signal.
At the transmitting end of the optical communication system according to the present invention, the aforesaid optical transmitting device includes an electrical signal modulating device for modulating the input data to an electrical signal, a laser signal generator electrically connected with the electrical signal modulating device for transforming the electrical signal into a laser signal, and a 2xc3x972 optical coupler optically connected with the electrical signal modulating device and the laser signal generator for receiving the laser signal and respectively transmitting the laser signal through a third optical path and a fourth optical path. Moreover, the first reference optical signal generator includes an automatic frequency controller (AFC) optically connected with the 2xc3x972 optical coupler through the fourth optical path for generating a voltage signal according to the frequency drift of the laser signal, wherein the voltage signal has an intensity variation following the frequency drift of the laser signal, and a first local oscillating laser signal generator electrically connected with the automatic frequency controller and optically connected with the 2xc3x972 optical coupler through a second optical path for generating a first reference laser signal and transmitting the first reference laser signal to the 2xc3x972 optical coupler through the second optical path, wherein the first reference laser signal frequency is varied with the intensity of the voltage signal generated by the automatic frequency controller so that the first reference laser signal has the first beat frequency difference with the laser signal.
At the receiving end of the optical communication system according to the present invention, the aforementioned optical receiving device includes a 2xc3x971 optical coupler optically connected with the optical transmission path and the second reference optical signal generator for receiving the carrier signal and the first reference optical signal through the optical transmission path, and receiving the second reference optical signal through a fifth optical path, a photodetector optically connected with the 2xc3x971 optical coupler through a seventh optical path for transforming the carrier signal and the first reference optical signal into an electrical signal, and an electrical signal demodulating device electrically connected with the photodetector for retrieving the input data and a third electrical signal having a third frequency difference for output. Furthermore, the second reference optical signal generator includes a filtering device electrically connected with the photodetector for retrieving a first electrical signal having a first frequency difference, a mixer electrically connected with the filtering device and the electrical signal demodulating device for mixing the first electrical signal and the third electrical signal to generate a reference signal having a frequency of the difference between the first beat frequency difference and the third frequency difference, a frequency discriminating device electrically connected with the mixer for generating a voltage signal having a frequency proportional to the frequency difference between said first beat frequency difference and said third frequency difference according to the difference between the first frequency difference and the third frequency difference, and a second local oscillating laser signal generator electrically connected with the frequency discriminating device and optically connected with the 2xc3x971 optical coupler through a sixth optical path for generating a second reference laser signal and transmitting the second reference laser signal to the 2xc3x971 optical coupler through the sixth optical path, wherein the second reference laser signal frequency can be adjusted according to the frequency drift of the carrier signal.
Remarkably, the optical transmission path is constructed with optical fiber, wherein the optical transmission path further includes a first telescope optically connected with the 2xc3x972 optical coupler through the third optical path for emitting the carrier signal and the first reference optical signal, a reflective mirror mounted on a satellite for reflecting the carrier signal and the first reference optical signal, and a second telescope optically connected with the 2xc3x971 optical coupler through the fifth optical path for receiving the carrier signal and the first reference optical signal reflected from the reflective mirror.
Taking as an example, the electrical signal modulating device and the electrical signal demodulating device performs the modulation/demodulation operation by using frequency-shift keying (FSK) modulation/demodulation method.
The optical communication system further includes an optical polarization controller optically connected between the first local oscillating laser signal generator and the 2xc3x972 optical coupler through the second optical path, and an optical polarization controller optically connected between the second local oscillating laser signal generator and the 2xc3x971 optical coupler through the sixth optical path.
In accordance with a second aspect of the present invention, a method for communication with an optical signal is provided and includes the steps of: (a) emitting a carrier signal carrying an input data, (b) emitting a first reference optical signal, wherein the first reference optical signal has a first beat frequency difference with the carrier signal, (c) receiving the carrier signal and the first reference optical signal from an optical transmission path, and (d) processing the carrier signal and the first reference optical signal with a second reference optical signal generated by a second reference optical signal generator, wherein the frequency of the second reference optical signal can be adjusted according to the first beat frequency difference and a second frequency difference between the carrier signal frequency and the second reference optical signal frequency so as to eliminate the frequency drifting effect of the carrier signal frequency.
Remarkably, the optical transmission path is constructed with optical fiber.